From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Joonsoo Kim , Russell King Subject: [ 60/86] ARM: 7643/1: sched: correct update_sched_clock() Date: Tue, 26 Feb 2013 16:08:08 -0800 Message-Id: <20130226235919.142149593@linuxfoundation.org> In-Reply-To: <20130226235912.881663118@linuxfoundation.org> References: <20130226235912.881663118@linuxfoundation.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: 3.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Joonsoo Kim commit 7c4e9ced424be4d36df6a3e3825763e97ee97607 upstream. If we want load epoch_cyc and epoch_ns atomically, we should update epoch_cyc_copy first of all. This notify reader that updating is in progress. If we update epoch_cyc first like as current implementation, there is subtle error case. Look at the below example. cyc = 9 ns = 900 cyc_copy = 9 == CASE 1 == write cyc = 10 read cyc = 10 read ns = 900 write ns = 1000 write cyc_copy = 10 read cyc_copy = 10 output = (10, 900) == CASE 2 == read cyc = 9 write cyc = 10 write ns = 1000 read ns = 1000 read cyc_copy = 9 write cyc_copy = 10 output = (9, 1000) If atomic read is ensured, output should be (9, 900) or (10, 1000). But, output in example case are not. So, change updating sequence in order to correct this problem. Signed-off-by: Joonsoo Kim Signed-off-by: Russell King Signed-off-by: Greg Kroah-Hartman --- arch/arm/kernel/sched_clock.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/arch/arm/kernel/sched_clock.c +++ b/arch/arm/kernel/sched_clock.c @@ -84,11 +84,11 @@ static void notrace update_sched_clock(v * detectable in cyc_to_fixed_sched_clock(). */ raw_local_irq_save(flags); - cd.epoch_cyc = cyc; + cd.epoch_cyc_copy = cyc; smp_wmb(); cd.epoch_ns = ns; smp_wmb(); - cd.epoch_cyc_copy = cyc; + cd.epoch_cyc = cyc; raw_local_irq_restore(flags); }